The Difference Between Being Sleepy and Fatigued (And Why It Matters)

Imagine going to a sleep specialist, neurologist, psychiatrist, or any doctor for that matter, and complain of feeling “tired.” Seems straightforward, right? Unfortunately, the word “tired” has several possible meanings, and the way I like to break them down is as follows: “Do you feel sleepy?” or “Do you feel fatigued?”

While they sound like the same exact thing, they are actually quite different. Sleepiness means you are ready and able to fall asleep – that is, you fall asleep in meetings, in class, or even while driving or stopped at a traffic light.

The causes for sleepiness can vary from something as simple as being sleep deprived, or to a more complicated issue such as obstructive sleep apnea or narcolepsy. Usually, when the underlying condition is eliminated, the sleepiness improves.

Fatigue, on the other hand, is more a feeling of not being your best. You are not necessarily falling asleep at all times of the day, but you are tired and don’t have that “get up and go” attitude about you. The causes for fatigue are so varied that we cannot mention them all here, but I will give you the biggest ones I see in my practice. Essentially, any condition that will leave you feeling unwell can lead to fatigue – depression is a huge one, along with anxiety, stress, vitamin deficiencies, or any chronic medical, neurological, or psychiatric condition.

The way we, as sleep practitioners, try to figure these issues out are with comprehensive sleep testing. An overnight sleep test, known as a polysomnogram (PSG), is extremely helpful to discover or rule out night-time sleep disorders.

We typically require more extensive testing to help us to evaluate and understand tiredness further. The extra testing we use in this case is a PSG followed by a next-day nap test, known as a multiple sleep latency test (MSLT). The MSLT allows us to objectively measure exactly how sleepy someone is. This nap testing consists of four to five 20-minute opportunities to sleep, spread throughout the day. This allows us to measure exactly how fast someone falls asleep and how deeply they sleep when they do.

The PSG/MSLT will allow us to diagnose disorders of sleepiness, such narcolepsy, but if someone is truly fatigued without being sleepy, the PSG/MSLT will typically come back normal.

If you feel tired, you should make a point to mention it to a doctor so you can investigate it. Depending on what the situation is, there may be direct evaluations and treatments (as in the case of excessive daytime sleepiness) or more roundabout ways to help, such as treating underlying issues like depression (as in the case of fatigue).

But either way, you owe it to yourself to feel good; and if not, to try to find out why.

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Daniel A. Barone, MD is currently an Assistant Professor of Neurology and the Associate Medical Director of the Weill Cornell Medical College Center for Sleep Medicine, where he sees patients with all forms of sleep disorders. He is the author of “Let’s Talk About Sleep,” a book written for the lay pubic to better understand sleep and sleep disorders, and what to do about them.